how download forms one at a time?

Are you referring to downloading your form submission one at a time? If yes, you can get a PDF format of the submission. This will download it one submission at a time.

https://www.jotform.com/help/73-How-to-Download-Form-Submissions-to-Excel-CSV-PDF

If you want to export individual submission to Excel, you can use the search filter box before exporting it to excel.

Example:

If you mean downloading form into fillable PDF, check this guide: https://www.jotform.com/help/212-How-to-Create-a-PDF-Form